The present invention relates to a power semiconductor switching element and, more particularly, to a semiconductor element having a low ON resistance.
Recently, power MOSFETs (power MOSFETs) have been widely used for power supplies in vehicles, power supplies for computer equipment, motor control power supplies, and the like. For these power supplies, importance is placed on efficiency and downsizing.
In switching power supplies that have been widely used, since power MOSFETs also serve as conventional diodes (synchronous rectification), the characteristics of power MOSFETs are very important. Two characteristics, ON resistance and switching speed, are especially important. As the ON resistance decreases, the energy consumed by a power MOSFET while a current flows decreases, and hence the efficiency of the power supply increases. As the switching speed increases, the switching frequency can be increased. This makes it possible to reduce the size of a magnetic circuit, e.g., a transformer. Therefore, the power supply can be reduced in size, and the efficiency of the magnetic circuit can be increased.
FIG. 44 is a sectional view of a conventional vertical power MOSFET.
As shown in FIG. 44, an n-type drift layer 112 is formed on one surface of an n-type semiconductor substrate 111 by epitaxial growth. P-type well layers 113 for MOS formation are selectively formed in the surface of the drift layer 112. N-type source layers 114 are selectively formed in the surfaces of the well layers 113. Trenches 115 are formed to reach the inside of the drift layer 112 from the surface of the source layers 114 through the well layers 113. Gate electrodes 119 are formed in the trenches 115 through silicon oxide films 118. In addition, a drain electrode 120 is formed on the other surface of the semiconductor substrate 111. Source electrodes 121 connected to the source layers 114 and well layers 113 are formed on the well layers 113.
Even in a case of ideal design, the characteristics of this type of power MOSFET are set in such a manner that the breakdown voltage and ON resistance must always satisfy the relationship defined by inequality (1). It has therefore been thought that any characteristics better than those defined by this relationship cannot be obtained.Ron<2.2×10−5 Vb2.25  (1)where Vb is the static breakdown voltage, and Ron is the ON resistance.
However, it has recently been reported that the upper characteristic limit can be exceeded by burying a p-type diffusion layer in the drift layer 112. According to a structure having this buried diffusion layer, the ON resistance certainly decreases. However, since the junction distance (area) is long (large), the junction capacitance is large, resulting in slow switching. For the same reason, too many carriers are injected into a reverse-conducting diode incorporated in an element, and hence the element tends to break during a period of reverse recovery.
In practice, therefore, the range of application of elements having such structures is limited. In addition, in forming an element, many epitaxial layers are formed by repeating epitaxial growth and ion implantation, resulting in an increase in cost.
As described above, in a conventional power MOSFET, it is difficult to decrease the ON resistance. Even if the ON resistance can be decreased, the switching speed decreases and the characteristics of a reverse-conducting diode deteriorate. Furthermore, a problem arises in terms of cost.